Boil the collected pollen in the pot at -3548, -781, -144 . You will be timed when this step updates!
You have 3 minutes to return to Kerr and hail him to apply the salve. The easiest way would be to take the zone travel horse from the New Combine Foothold to the Sanctuary of Mourning and jump off when you're near Kerr.
Speak to Kerr at the Sanctuary of Mourning (the broken stele) to complete the quest. ( -1786, -599, -1068 )